The long-term goal of this research is to understand the spatial organization of motor units with muscles, and how motor unit activation patterns are determined for multi-functional muscles. These issues are relevant to the rehabilitation of individuals with neurologic impairments, such as stroke and spinal cord injury. We first address basic issues of coordination in normal intact muscles, namely the link between recruitment patterns and multi-functional output. The proposed experiments test hypotheses about the nature of this link by characterization motor unit recruitment patterns in various isometric tasks, by characterizing the forces produced by individual motor units, and by testing for specific constraints between motor units within a muscle. The specific aims of this project are: 1. To examine the relation between activation and mechanical actions of motor units in the first dorsal interosseus, a muscle that is multi- functional and is well-suited to characterization of individual motor unit forces using spike-triggered averaging. 2. To examine the relation between motor unit recruitment, spatial distribution and spatial mechanical actions in the human deltoid, a muscle that clearly exhibits task- and location-dependent recruitment patterns, and has a diverse range of mechanical functions. 3. To evaluate potential mechanisms for task dependent organization of the motor units in a multi-functional muscle, specifically evidence for correlated activation of units with similar mechanical functions that varies in an elderly and continuous manner with the degree to which a unit contributes mechanically to a task. An understanding of how motor units are coordinated in able-bodied individuals is a first step in improving rehabilitation techniques following injury.